1. Field of the Invention
The present invention relates to a speed measuring apparatus for measuring the speed of a moving object or fluid and, more particularly, to a laser Doppler velocimeter for detecting speed by detecting the shift of the frequency of laser light.
2. Description of the Related Art
Hitherto, as an apparatus for measuring the moving speed of an object or fluid in a non-contact manner and with a high degree of accuracy, a laser Doppler velocimeter has been used. A laser Doppler velocimeter is an apparatus which projects laser light onto a moving object or fluid to measure the moving speed of the moving object or the moving fluid by using the effect such that the frequency of the light scattered by the moving object or the moving fluid shifts proportionally to the moving speed (the Doppler effect).
FIG. 1 is an illustration of an example of a conventional laser Doppler velocimeter.
In the figure, the laser light emitted from a laser 1 is made into a parallel beam 3 by a collimator lens 2, and divided into two light beams 5a and 5b by a beam splitter 4. After the two beams are reflected by mirrors 6 and 6', the light beams enter an object or fluid 7 moving at a speed V at an incidence angle .theta.. The light scattered by the object or fluid is detected by a photodetector 9 through a light-condensing lens 8. At this time, the frequency of the scattered light caused by the two light beams is subjected to Doppler shifts of +.DELTA.f and -.DELTA.f, respectively, in proportion to the moving speed V. In this example, if the wavelength of the laser light is denoted as .lambda., .DELTA.f can be expressed by the following equation (1): EQU .DELTA.f=Vsin.theta./.lambda. (1)
The scattered lights which are subjected to the Doppler shifts of +.DELTA.f and -.DELTA.f interfere each other, causing light and dark changes on the photosensitive surface of the photodetector 9, and the frequency F thereof (the beat frequency) is given by the following equation (2): EQU F=2.DELTA.f=2Vsin.theta. (2)
Therefore, if the frequency (hereinafter referred to as the Doppler frequency) of the output signals of the photodetector 9 is measured, it is possible to measure the speed V of the moving object or moving fluid (an object to be measured) on the basis of equation (2).
In such a laser Doppler speed meter as in the conventional example, as is clear from equation (2), the Doppler frequency F is inversely proportional to the wavelength .lambda. of the laser. Therefore, a laser light source in which the wavelength is stable must be used for a laser Doppler velocimeter. For a laser light source in which continuous oscillation is possible and the wavelength is stable, a gas laser using He-Ne or the like is well known. However, in such a laser, the laser oscillator is large, and high voltage is required for the power supply. Thus, the apparatus is large and expensive. Also, although laser diodes (semiconductor laser) used in compact discs, video discs, optical fiber communications or the like are ultra-small and can be driven easily, such laser diodes are dependent upon temperature, which is problematical.
FIG. 2 shows an example of the standard dependence of a laser diode upon temperature (cited from "'87 Mitsubishi Semiconductor Data Book: Photosemiconductor Elements"). The portion where the wavelength varies continuously is caused mainly by temperature-dependent changes of the reflective index of the active layer of the laser diode, which is from 0.05 to 0.06 nm/.degree. C., while the portion where the wavelength varies non-continuously is called vertical mode hopping, which is from 0.2 to 0.3 nm/.degree. C.
To stabilize the wavelength, generally, a method of controlling the laser diode at a constant temperature is used. In this method, temperature control members such as a heater, a radiator, or a temperature sensor, must be mounted in the laser diodes in such a way that only small heat resistance occurs, and the temperature must be controlled precisely. The laser Doppler velocimeter is relatively large, and the cost thereof is increased, and instability due to the above vertical mode hopping cannot be avoided completely.
As a laser Doppler velocimeter which solves the above-described problem, a method has been disclosed in Japanese Patent Laid-Open No. 2-262064 in which laser light serving as a light source is made to enter a diffraction grating, two diffraction lights of +n-th and -n-th (n: 1, 2, . . . ) order other than 0-th order are projected to a moving object or moving fluid at the same intersection angle as the angle formed by the two light beams, and the light scattered from the moving object or moving fluid is detected.
FIG. 3 shows an example in which laser light I is made to enter the transmission type diffraction grating 10 having a grating pitch d perpendicularly to the direction t in which the grating is orientated, and the diffraction angle .theta.0 is given by the following equation: EQU sin.theta.0=m.lambda./d
where m is the diffraction order (0, 1, 2, . . . ), and .lambda. is the wavelength of light.
Of these, .+-.nth-order light other than 0-th order is expressed by equation (3): EQU sin.theta.0=.+-.n.lambda./d (3)
where n is 1, 2, . . . .
FIG. 4 shows a case in which two light beams of .+-.nth-order light are projected onto an object 7 to be measured by the mirrors 6 and 6' disposed in parallel to each other so that the incidence angle becomes .theta.0. The Doppler frequency F detected by the photodetector 9 becomes as follows on the basis of equations (2) and (3): EQU F=2Vsin.theta.0/.lambda.=2nV/d
(4)
Thus, the Doppler frequency F does not depend upon the wavelength X of the laser light I, and is inversely proportional to the wavelength .lambda. of the laser light I and proportional to the speed of the object 7 to be measured. Since the grating pitch d can be made to be sufficiently stable, the Doppler frequency F, which is proportional to the speed of the object 7 to be measured, can be obtained. The same applies if a reflection type diffraction grating is used for the diffraction grating 10.
Also, if light having high coherent characteristics, such as a laser, is projected onto an object, generally, the light is scattered by fine irregularities on the surface of the object and is subjected to random optical-wave phase modulation, and a spotted pattern, what is commonly called a speckle pattern, is formed on the observed surface. In the laser Doppler velocimeter, when an object to be measured moves, a change in light and dark due to the Doppler shift on the photosensitive surface of the photodetector is modulated into irregular light and dark signals by the flow of the speckle pattern, and also the output signals of the photosensor are modulated by the change in the reflectance of the object to be measured.
In the above laser Doppler velocimeter, since the frequency of the light and dark modulation by the flow of the speckle pattern and the frequency of the reflectance of the object to be measured are generally a low frequency in comparison with the Doppler frequency shown by the above equation (4), a method is used in which the low frequency components of the output signals are removed by a high pass filter, and only the Doppler signals are taken out. However, when the moving speed of the object to be measured is slow and the Doppler frequency is low, the difference between the low frequency and the Doppler frequency becomes small, the high pass filter cannot be used, and the moving speed of the object cannot be measured, which is problematical. The speed direction cannot be detected from a point of view of principles.
Accordingly, an apparatus for measuring at a low speed, shown in FIG. 5, is disclosed (Japanese Patent Laid-Open No. 3-235060). The diffraction grating 10 having a grating pitch d is disposed on the cylinder 11 and is rotated in the direction of the arrow at a peripheral speed Vg. When the laser light is made to enter a moving diffraction grating, the laser light is divided into .+-.nth-order diffraction lights 5 and 5', each of which receives positive and negative Doppler shifts .+-.nVg/d, respectively, and the diffraction angle .theta.0 satisfies the following: EQU sin.theta.0=nX/d (5)
where .lambda. is the wavelength of the light. When the two light beams of the .+-.nth-order light are projected onto the object 7 to be measured moving at a speed V by the mirrors 6 and 6' disposed in parallel to each other so that the incidence angle becomes .theta.0, of the light scattered from the object 7 to be measured, +n-th order light 5a receives a Doppler shift of n(Vg+V)/d, and -n-th order light 5a receives a Doppler shift of -n(Vg+V)/d, and interfere with each other. The Doppler frequency F becomes : EQU F=2n (Vg+V)/d. (6)
Thus, a Doppler frequency which is not dependent upon the wavelength of the laser light can be obtained. That is, even if the speed of the object 7 to be measured is slow, the frequency difference between the low frequency components arising from the change in light and dark modulation due to the flow of a speckle pattern or from the change in the reflectance of the object to be measured can be assumed sufficiently on the basis of the moving speed Vg of the diffraction grating. Removing the low frequency components of the output signals by a high pass filter and taking out only Doppler signals makes it possible to detect speed.
FIGS. 6 (a) and 6 (b) show the relationship between the speed V of the object to be measured and the Doppler frequency F in a laser Doppler velocimeter employing .+-.first-order diffracted light of the diffraction grating; FIG. 6(a) shows a case in which the diffraction grating is fixed; and FIG. 6 (b) shows a case in which the diffraction grating is moving at a speed Vg, where Fg=2Vg/d.
As can be seen in the figures, in FIG. 6 (a), even if a certain frequency F1 is detected, it is not possible to determine the direction of the movement because two speeds V1 and -V1, the directions of which are different, correspond to each other. On the other hand, in FIG. 6 (b), the Doppler frequency F=Fg+F1 can be obtained for speed V1, and the Doppler frequency F=Fg-F1 can be obtained for speed -V1, and the direction of the speed can also be detected.
That is, if the moving speed Vg of the diffraction grating is made constant, the following relation is satisfied on the basis of equation (6) : EQU V=Fd/2-Vg (7)
Thus, it is possible to measure the speed V by detecting the Doppler frequency F.
In the construction shown in FIG. 4, it is necessary that the light beam before entering the diffraction grating be a collimated light beam in order to form the construction in which the collimated light beam is projected onto the object to be measured. In the apparatus of FIG. 4, when the grating pitch of the diffraction grating 10 is determined, the interval of the interference fringes formed by this grating is fixed.
In the construction of FIG. 6, a diffraction grating is disposed on the cylinder. Various methods of manufacturing the diffraction grating, and the materials therefor may be conceivable. When the diffraction grating is to be formed at a lower price, a plastic material formed by resin molding is preferable. However, since a plastic material has a very large thermal expansion, the grating pitch d of the diffraction grating on the cylinder varies with temperature. That is, since the grating pitch d handled as the constant in equation (7) varies, even if the Doppler frequency F is detected, it becomes difficult to accurately measure the speed V.